The present invention relates to ultramicrotomy and, more particulary, to an ultramicrotome for improved sectioning of specimens. An ultramicrotome is a device for sectioning thin slices of specimen material for electron microscopic analysis. Typically, organic specimen material is embedded in a block of epoxy after being heavily stained with uranium and osmium. Thin slices, or sections, on the order of 1000 Angstrums in thickness, are then made of the specimen block using the ultramicrotome. Production of such thin sections with the ultramicrotome remains somewhat of an art and requires a high degree of skill. This sectioning operation typically is viewed by the ultramicrotome operator through a microscope mounted on the device.
The ultramicrotome typically includes a chuck for holding the specimen block. Cutting of the block is accomplished with a diamond knife which is held stationary as the specimen block is moved into cutting contact. Glass knives may be used for only a few sections before they lose their sharpness and must be discarded; for this reason glass knives are generally used only by students. Diamond edge knives, while expensive, maintain a sharp edge during extended periods of use and are, therefore, preferred for most applications. The diamond knife is held by an assembly includinga knife boat which holds a quantity of fluid. The diamond knife extends across one edge of the knife boat and the boat may be filled with water so that the water extends just up to the cutting edge of the knife.
The specimen chuck is typically fastened to the end of a cantilever arm connected to a linkage which may be driven either by a hand crank or by a small motor. The linkage is arranged such that the specimen block is moved downward and into contact with the knife edge. After a section is cut, specimen block is moved laterally and returned to a position above the knife edge in preparation for the next downward cutting stroke. Between each downward cutting stroke, the specimen block and chuck assembly is advanced a predetermined distance toward the knife such that sections of the desired thickness are cut. This advancement arrangement may be either a mechanical system in which a threaded rod connected to the cantilever arm is rotated through a predetermined angle, or a thermal system in which the temperature of a rod having known thermal expansion characteristics is altered slightly during each cutting stroke.
The thin sections which are produced by the ultramicrotome are floated onto the surface of the fluid in the knife boat as they are cut in order to prevent them from being crumpled or otherwise destroyed. It is therefore important that the fluid surface extend up to the cutting edge of the diamond knife. It is very important, however, that this fluid at no time migrate over the knife edge surface and onto the outer edge of the knife. If this occurs, the specimen block will be wetted during each cutting stroke and successful sectioning will become impossible.
The problem of wetting the outer surface of a diamond knife is relatively common. In the operating instruction manual for the SORVALL MT2-B "Porter Blum" ultramicrotome, the following comment is made:
"Sometimes one encounters a situation in which the fluid in the trough seems to `jump` onto the block face as it passes the knife, making sectioning difficult, if not impossible. This may be due to a drop of fluid on the vertical face of the knife (toward the specimen), and in this case can be cured by removing this fluid with a piece of the lens tissue. If this doesn't work, lowering the level of the fluid in the trough may help. If this makes it difficult to see the sections as they are cut, move the microscope forward and the lamp closer to the panel of the microtome and tip the microscope to recover the correct image of the knife and sections. It may even be necessary to sacrifice the image of the section nearest the knife edge to get the fluid level low enough to prevent wetting of the block face."
It is seen, therefore, that a need exists for an improved ultramicrotome device in which the migration of the fluid over the edge of the knife is eliminated, thereby permitting move uniform sectioning.